1. Field of the Invention
The present invention relates to a thin film transistor (TFT), a method for manufacturing the TFT, and apparatuses using the TFT.
2. Description of the Related Art
Generally, TFTs are manufactured on an insulating substrate by using a hydrogen-passivated amorphous silicon technology and a polycrystalline silicon technology.
According to the hydrogen-passivated amorphous silicon technology, since the maximum temperature during the manufacturing steps thereof is low, i.e., about 300.degree. C., the mobility of carriers is low,i.e., about 1 cm.sup.2/V.multidot.sec. Also, an insulating substrate can be a glass substrate having a low melting temperature, which would decrease the manufacturing cost.
However, in an active matrix-type liquid crystal display (LCD) apparatus manufactured by the hydrogen-passivated amorphous silicon technology, TFTs manufactured by the hydrogen-passivated amorphous silicon technology are used as the switching elements of pixels of a display panel of the LCD apparatus, and the TFTs are driven by a driver of an integrated circuit apparatus connected to a periphery of the display panel. As a result, since the display panel is connected to the driver for driving the display panel by a tape automated bonding (TAB) process or a wire bonding process, if the connection pitch between the display panel and the driver becomes small, it is actually impossible to connect the display panel to the driver.
On the other hand, according to the polycrystalline silicon technology, since the maximum temperature during the manufacturing steps thereof is high, i.e., about 1000° C., the mobility of carriers is about 30 to 100 cm2V sec. For example, a high temperature annealing process is required to convert amorphous silicon into polycrystalline silicon. Also, if TFTs manufactured by the polycrystalline silicon technology are used as the switching elements of pixels of a display panel of an active matrix-type LCD apparatus, a driver for driving the display panel can also be formed on the same substrate of the display panel, so that the above-mentioned TAB or wire bonding process is unnecessary.
In the polycrystalline silicon technology, since the maximum temperature is high as stated above, the insulating substrate has to be a fused quart substrate having a high melting temperature, for example. This would increase the manufacturing cost.
In order to decrease the manufacturing cost, i.e., in order to decrease the temperature for converting amorphous silicon into polycrystalline silicon to adopt a glass substrate having a low melting temperature, a laser technology has been combined with the polycrystalline silicon technology.
In a prior art method for manufacturing a TFT by the polycrystalline silicon technology combined with the laser technology, first, a substrate covering layer made of silicon oxide is deposited on a glass substrate by a low pressure chemical vapor deposition (LPCVD) process or the like. Next, an amorphous silicon (a-Si) layer is deposited on the substrate covering layer by an LPCVD process or the like. Next, the amorphous silicon layer is irradiated with a laser beam by moving the glass substrate along X- and Y-directions. This will be explained later in detail.
In the above-described prior art method, however, the laser beam has a rectangular size of several millimeters or several hundred micrometers. Additionally, the energy of the laser beam is relatively low, for example, 300 mJ/cm.sup.2, and also, the slope of the energy with respect to the X- or Y-direction is relatively gentle. As a result, the amorphous silicon layer is converted into a polycrystalline silicon layer which has a randomly-small grain size. Thus, since the polycrystalline silicon layer forming a source region, a channel region and a drain region has a randomly-small grain size, the mobility of carriers is so low that the ON-current is low.